DESCRIPTION: Keratinocytes at sites of inflammation express class II major histocompatibility complex (MHC) molecules in response to IFN-gamma. Expression of class II MHC allows keratinocytes to display peptide-MHC II complexes and function as antigen-presenting cells (APC) for CD4 T cells. Previous studies have indicated that class II positive keratinocytes are tolerogenic APC, but the effects of keratinocyte antigen presentation on activation of the Th1 and Th2 T cell subsets have not been addressed. The central hypothesis to be tested is that antigen presentation by class II positive epithelial cells has distinct effects on an TH1 and Th2 cell subsets, with Th1 cells developing anergy and Th2 cells responding by production of cytokines (i.e. IL-4 and IL-10) that further inhibit Th1 function. Antigen presentation to T cells by class II positive keratinocytes can also be influenced by costimulatory molecules from the B7 family, and it has been proposed (but not proven using in vivo models) that B7-1 and B7-2 have opposing effects on the differentiation of CD4 T cells. To further understand how keratinocyte expression of class II MHC and costimulatory molecules affects induction of T cell immunity in intact skin, the following specific aims are proposed: (1) to determine how Th1 and Th2 responses to cutaneous antigens are modulated by antigen presentation on transgenic epidermal keratinocytes constitutively expressing class II MHC antigens. Constitutive expression of class II MHC in murine basal keratinocytes will be achieved by expression of transgenes encoding both chains of a mouse class II MHC molecule under the control of the human K14 promoter. The development of hapten-specific T cell immunity after epicutaneous sensitization will be analyzed by comparing contact hypersensitivity (CHS) responses, T cell cytokine synthesis, and anti-hapten antibody production in transgenic mice and controls. (2) To determine if class II positive keratinocytes alone (in the absence of class II positive Langerhans cells and dendritic cells) can initiate CD4 T cell responses to cutaneous antigens. Transgenes permitting constitutive expression of class II MHC by keratinocytes will be bred onto class II null mice, specifically reconstituting class II expression on keratinocytes, but no other cutaneous APC. (3) To determine whether the CD28 counterligand (B7-1 vs. B7-2 vs. B7-3) expressed by keratinocytes influences relative activation of Th1 or Th2 T cells during CHS responses. The animal models generated during these studies will be uniquely powerful in defining the molecular mechanisms by which epithelial APC expressing class II MHC promote oral tolerance and other forms of peripheral tolerance.